1. Field of the Invention
The present invention relates to a wire electric discharge machine, and in particular to a manual feeding device for feeding axes manually to move a workpiece relatively to a wire electrode.
2. Description of Related Art
A wire electric discharge machine machines a workpiece by relatively moving the workpiece to a wire electrode stretched and applying a voltage between the wire electrode and the workpiece to cause electric discharge between the wire electrode and the workpiece. During machining, the workpiece is relatively moved to the machining electrode according to a machining program. However, in preparation before starting machining, such as moving the workpiece to a machining start position, the workpiece or the electrode needs to be moved by manual operation.
In order to relatively move the workpiece to the wire electrode, there are provided mutually perpendicular X, Y and Z axes, and U and V axes for moving one of the wire guides in mutually perpendicular U-axis and V-axis directions to tilt the wire electrode to perform taper machining.
Generally, there are provided X and Y axes for moving a table (workpiece) in X-axis and Y-axis directions, a Z axis for moving an upper guide in a Z-axis direction perpendicular to the X and Y axes, and U and V axes for moving the upper guide in the X-axis and Y-axis directions.
In manual feeding, axis motion keys for the X, Y, Z, U and Z axes are operated to move the axes in the axis directions corresponding to the keys operated. The manual operation includes a jog feed in which an axis is driven continuously while an axis motion key is operated, and a step feed in which an axis is moved by a predetermined amount each time an axis motion key is operated. The jog feed is used to place an axis roughly in an intended position, while the step feed is used to place an axis exactly in an intended position.
Selection between the jog feed and the step feed is made by a selection switch. When the jog feed mode is selected and an axis motion key is operated, the workpiece is relatively moved to the wire electrode in an axis direction corresponding to the operated axis motion key at a predetermined speed. When the step feed is selected and an axis motion key is operated, the workpiece is moved relatively to the wire electrode by a predetermined amount.
There is known an numerical control unit in which speed control in the jog feed is performed such that when the time for which a jog feed key has been operated exceeds a predetermined length of time, the feeding speed is increased by a predetermined amount (see JP 2-17506A).
When a feed axis is driven by manual operation to place it in a machining start position, for example, first the feed axis is moved close to the intended position by the jog feed, and then moved exactly to the intended position by the step feed. Hence, manual operation is performed first in the jog feed mode, and then switched to the step feed mode so that the feed axis can be driven by the step feed. Thus, mode change is needed in the same manual operation, which is inconvenient.